1) Field of the Invention
The invention relates generally to photolithography processes and equipment for photolithography, and more particularly relates to a method for optimizing the conditions of illumination of a lithographic apparatus.
2) Description of the Prior Art
Photolithography is a well-known process for transferring geometric shapes present on a photomask onto a resist layer over a substrate. In lithographic processing, a photosensitive polymer film called photoresist is normally applied over a substrate and then allowed to dry. An exposure tool is utilized to expose the wafer with the proper geometrical patterns through a mask by means of a source of light or radiation. After exposure, the wafer is treated to develop the mask images transferred to the photosensitive material. These masking patterns are then used to create the device features of the circuit.
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device (or patterning structure), which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A simplified diagram of a conventional exposure tool is shown in FIG. 1. As can be seen, light source 1100 projects light waves 1108 through opening 1102 in aperture stop 1101. Opening 1102 is commonly referred to as the aperture or pupil of the aperture stop. Condenser lens 1105 collects the light from the opening 1102 and focuses it on mask 1106 such that the mask 1106 is evenly illuminated. When illuminating beam 1103 passes through mask 1106, imaging beam 1109 is generated. Imaging beam 1109 is projected through projection lens 1107 such that the image of the pattern on the mask 1106 is focused onto the resist 1111 over the substrate or wafer 1110. As can be seen in FIG. 1, the opening 1102 is situated in the center of aperture stop 1101. Because of this, illuminating beam 1103 is projected along the optical axis (dashed line 1104) from the opening 1102 to condenser lens 1105 and mask 1106. This type of illumination method is called “On-axis illumination,”—the name implying that the illumination beam is “on” the optical axis.
One of the goals in integrated circuit fabrication is to faithfully reproduce the original design on the substrate (via the mask). As the demand to image smaller and smaller features in the semiconductor manufacturing process has continued unabated, the limitations of optical lithography that were once accepted have been exceeded repeatedly.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution R as shown in equation (a):1R=k1*λNA  (a)where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system and k1 is a Rayleigh constant (process dependent adjustment factor).
It follows from equation (a) that the resolution can be improved in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1. All of these strategies have been pursued simultaneously in the past and are expected to continue to be pursued in the future.
The performance of a lithographic apparatus and its limitation may also be explained and characterized with the depth of focus (DOF), which is generally viewed as one of the most critical factors in determining the resolution of the lithographic projection apparatus. The DOF, defined in equation (b), is defined as the distance along the optical axis over which the image of the pattern is adequately sharp.DOF=+/−k2*λNA2  (b)where k2 is an empirical constant.
Additional important responses/measures that provide more insight into the real difficulties associated with photolithography at the resolution limit include the exposure latitude (EL), the dense: isolated bias (DIB), and the mask error enhancement factor (MEEF). The exposure latitude describes the percentage dose range where the printed pattern's critical dimension (CD) is acceptable, typically 10%. It is used along with the DOF to determine the process window, i.e. the regions of focus and exposure that keep the final resist profile within prescribed specifications. As for the DIB, it is a measure of the size difference between similar features, depending on the pattern density. Finally, the MEEF describes how mask CD errors are transmitted into substrate CD errors.
As the semiconductor industry moves into the deep submicron regime, the resolution limit of currently available lithographic techniques is being reached due to a decrease in the depth of focus, difficulty in the design of projection system and complexities in the projection system fabrication technology. In order to address this issue, there have been continued endeavors to develop resolution enhancement techniques.
Historically, the resolution limit of a lithographic projection apparatus was optimized by the control of the relative size of the illumination system numerical aperture (NA). Control of this NA with respect to the projection system's NA allows for modification of spatial coherence at the mask plane, commonly referred to as partial coherence σ. This is accomplished, for example, through specification of the condenser lens pupil in a Köhler illumination system. Essentially, this allows for manipulation of the optical processing of diffraction information. Optimization of the partial coherence of a projection imaging system is conventionally accomplished using full circular illumination apertures. By controlling the distribution of diffraction information in the projection system with the illuminator pupil size, maximum image modulation can be obtained. Illumination systems can be further refined by considering variations to full circular illumination apertures. A system where illumination is obliquely incident on the mask at an angle so that the zero-th and first diffraction orders are distributed on alternative sides of the optical axis may allow for improvements. Such an approach is generally referred to as off-axis illumination.
Off-axis illumination improves resolution by illuminating the mask with radiation that is at an angle to the optical axis of the projection system. The angular incidence of the radiation on the mask, which acts as a diffraction grating, improves the contrast of the image by transmitting more of the diffracted orders via the projection system. Off-axis illumination techniques used with conventional masks produce resolution enhancement effects similar to resolution enhancement effects obtained with phase shifting masks.
Various other enhancement techniques that have been developed to increase the resolution and the DOF include optical proximity correction (OPC) of optical proximity errors (OPE), phase-shift masks (PSM), and sub-resolution assist features (SRAF). Each technique may be used alone, or in combination with other techniques to enhance the resolution of the lithographic projection apparatus.
Currently available illumination intensity distributions or arrangements include small, or low, sigma conventional, annular, quadrupole, and QUASAR, with the illuminated areas (hereinafter referred to as the aperture(s)). The annular, quadrupole and QUASAR illumination techniques are examples of off-axis illumination schemes. Quadrapole Illumination—a light source where light is split through four round openings located away from the optical axis of an illumination system. Quadrapole illumination is a type of off axis illumination and improves the minimum feature that an exposure system can resolve for a given wavelength.
Small sigma illumination is incident on the mask with approximately zero illumination angle (i.e. almost perpendicular to the mask) and produces good results with phase shifting masks to improve resolution and increase the depth of focus. Annular illumination is incident on the mask at angles that are circularly symmetrical and improves resolution and increases depth of focus while being less pattern dependent than other illumination schemes. Quadrupole and QUASAR illumination are incident on the mask with four main angles and provide improved resolution and increased depth of focus while being strongly pattern dependent.
There is a challenge to improve the quality and reduce the size of the masking patterns.